fix: disable cache_control injection for third-party providers (#276)

getPromptCachingEnabled() returned true for all providers including
Azure Foundry, OpenAI, Gemini, and GitHub. This caused cache_control
blocks to be injected into every request sent to 3P endpoints.

Azure Foundry's strict Responsible AI content filter treats unexpected
Anthropic-specific fields (cache_control: { type: "ephemeral" }) as
a jailbreak signal and rejects the request with a 400 error — even
for a simple prompt like "hi".

Fix: return false early when provider is not firstParty, bedrock, or
vertex — the only providers that understand and support prompt caching.

Fixes #273
Related: #267 (Finding 1)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

src/services/api/claude.ts

@@ -331,6 +331,14 @@ export function getExtraBodyParams(betaHeaders?: string[]): JsonObject {
 }
 
 export function getPromptCachingEnabled(model: string): boolean {
+  // Prompt caching is an Anthropic-specific feature. Third-party providers
+  // do not understand cache_control blocks and strict backends (e.g. Azure
+  // Foundry) reject or flag requests that contain them.
+  const provider = getAPIProvider()
+  if (provider !== 'firstParty' && provider !== 'bedrock' && provider !== 'vertex') {
+    return false
+  }
+
   // Global disable takes precedence
   if (isEnvTruthy(process.env.DISABLE_PROMPT_CACHING)) return false